Method for actively controlling electric potential at the head/disk interface of a magnetic recording disk drive

ABSTRACT

An electrical potential difference between a slider body and a hard disk of a hard disk drive is eliminated based on the flying-height spacing of the slider body between the slider body and the hard disk. A predetermined bias voltage is applied between the slider body and the hard disk that includes a DC component and an AC component and that is based on the detected flying-height spacing of the slider body. The flying-height spacing can be detected based a minimum slider-to-disk clearance change from a design flying height of the slider at a frequency of the AC component as the DC component of the predetermined bias voltage is varied. Alternatively, the flying-height spacing can be detected based on a minimum electrodynamic response of the slider to a first harmonic of the AC frequency of the AC component as the DC component is varied.

CROSS-REFERENCED TO RELATED APPLICATION

The present application is related to co-pending and co-assigned patentapplication Ser. No. ______ (Attorney Docket No. HSJ9200300148US1),entitled “Magnetic Recording Disk Drive With Actively ControlledElectric Potential At The Head/Disk Interface,” which is filedconcurrently herewith and is incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to hard disk drives (HDDs). Moreparticularly, the present invention relates to a technique for detectingand eliminating an electrical potential difference between a slider bodyand a disk surface of an HDD, such as a contact potential caused bymaterial differences between the slider and the disk or a potentialgenerated by tribocharging.

2. Description of the Related Art

FIG. 1 shows an exemplary hard disk drive (HDD) 100 having a dual-stageservo system for positioning a slider assembly 101 over a selectedconcentric data information track on a magnetic disk 102 for writingdata to and/or reading data from the selected track. The dual-stageservo system of HDD 100 includes a primary actuator 104, such as arotary voice-coil motor (VCM), for coarse positioning an actuator arm105 and a read/write head suspension 106, and a secondary actuator (notshown in FIG. 1), such as a microactuator or micropositioner, for finepositioning slider assembly 101 over a selected track. A microactuator,as used herein, is a small actuator that is placed between a suspensionand a slider and moves the slider relative to the suspension. Sliderassembly 101 includes a read/write head (not shown in FIG. 1) having aread element, such as a Giant Magnetoresistive (GMR) element, and awrite element that respectively read data from and write data to aselected data track. While HDD 100 is shown as having only a singlemagnetic disk 102, HDDs typically have a plurality of stacked, commonlyrotated, rigid magnetic disks and a corresponding number of actuatorarms, read/write head suspensions, secondary actuators and sliderassemblies.

As slider-to-disk spacing becomes smaller than 10 nm, electrostatic andintermolecular forces between a slider and a disk become increasinglysignificant. Even when a slider body and a disk are both grounded, apotential difference can exist between the slider body and the disk thatcan generate an electrostatic force greater than the van der Waalsforce. One source of the potential difference is tribocharging, orfrictional electrification of non-conducting materials on the sliderbody. (See, for example, J. D. Kiely et al., “Tribocharging of themagnetic hard disk drive head-disk interface,” Journal of AppliedPhysics, Vol. 91, No. 7, pp. 4631-4636, Apr. 1, 2002.) Another source isthe contact potential between the conducting materials of the sliderbody and the disk.

FIG. 2 depicts an electrostatic field that can exist between a sliderbody 201 and a hard disk 202 of a hard disk drive. A suspensionsupporting slider body 201 is not shown. As disk 202 rotates, disk 202moves from right to left with respect to slider body 201, as indicatedby arrow 203. Enlargement 204 of the slider-disk interface shows linesrepresenting an electric field 205 that is formed from a potentialdifference between slider body 201 and disk 202. Electric field 205exists between slider body 201 and disk 202 all along the length ofslider body 202, but is only indicated in enlargement 204 because theintensity of electric field 205 is greatest at the trailing edge ofslider body 201. As mentioned, one source for the potential differenceis the contact potential that originates from the conducting portions ofthe slider body and disk having different work functions and fromtribocharging of the non-conducting portions. Another source for thepotential difference is tribocharging associated with the spindle motorbearing, which can shift the disk potential significantly from groundpotential.

Electric field 205 between slider body 201 and disk 202 generates anelectrostatic force that acts the on the surfaces of slider body 201 anddisk 202 within the slider-disk interface. The electrostatic forceassociated with electric field 205 depends on the applied voltage andthe other physical parameters of the slider-disk interface. As thepotential difference between the slider body and the disk increases, theslider flying height is reduced from the design flying height of theslider.

For example, FIG. 3 shows the flying height (FH) for a high-pitch airbearing design and a low-pitch air bearing design as a functionslider-disk potential difference. Both air bearing designs have a designfly height 9 nm above a disk. The high-pitch slider has a dynamic pitchangle of 180 μrad. The low-pitch slider has a dynamic pitch angle of 70μrad at a flying height of 9 nm. Curve 301 represents the flying heightfor the high-pitch slider as a function of the slider-disk potentialdifference. Curve 302 is the flying height for a low-pitch slider as afunction of the slider-disk potential difference. The low-pitch slideris more sensitive to an applied DC electric field than a high-pitchslider because a low pitch design has on average a smaller separationdistance than a high-pitch design when the electrostatic force isintegrated over the slider. The flying height is reduced by about 0.5 nmfor a potential difference of about 0.5 V for both the high-pitch andlow-pitch air bearing designs. The value of the flying height at 2.5 Vfor the low-pitch slider is extrapolated to 0 nm because contact occursat 2.5 V.

In current hard drives, the disks are lubricated usingperfluoropolyether-type lubricants having long polar chains, such asFomblin Z-DOL. The polarity of the lubricant in the presence of anelectrostatic field at the head-disk interface can cause lubricant to beremoved from the disk surface and possibly be depleted from the disksurface. The removed lubricant is usually picked up by the slider andcan lead to drive failure. Consequently, as lubricant is removed, thelubricant is attracted to slider body 201 by the polarity of thelubricant and the electrostatic force and becomes concentrated in areason slider body 201 that have a high electric field, such as the trailingedge of the slider body. Once concentrated, the lubricant can formdroplets that can lead to read/write errors when the droplets oflubricant enter the head-disk interface. Electric field 205 can alsoattract particles onto slider body 201 that can lead to scratches onboth slider body 201 and the surface of disk 202. Yet another potentialproblem that is caused by the electrostatic force is an increasedvibrational coupling between slider body 201 and disk 202, leading tolarger modulations of the slider-disk spacing.

A number of researchers have found over the past years thatslider-to-disk spacing can be controlled by adjusting a DC bias appliedto the slider-disk interface. See, for example, U.S. Pat. No. 6,005,736to E. T. Schreck; U.S. Pat. No. 6,529,342 to Z. Feng et al.; and U.S.Pat. No. 6,366,416 B1 to D. W. Meyer et al.

Accordingly, what is needed is a technique for determining the optimumbias voltage that should be applied between a slider body and a disk foreliminating an electrical potential difference that exists between theslider body and the disk, such as a contact potential.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a technique for determining the optimumbias voltage that should be applied between a slider body and a disk foreliminating an electrical potential difference that exists between theslider body and the disk, such as a contact potential.

The advantages of the present invention are provided by a system and amethod for eliminating an electrical potential difference between aslider body and a hard disk of a hard disk drive. According to thepresent invention, a flying-height spacing of the slider body betweenthe slider body and the hard disk is detected. A predetermined biasvoltage is applied between the slider body and the hard disk thatincludes a DC component and an AC component and that is based on thedetected flying-height spacing of the slider body. One aspect of thepresent invention provides that the flying height spacing of the sliderbody is detected based on determining a minimum slider-to-disk clearancechange from a design flying height of the slider at a frequency of theAC component as the DC component of the predetermined bias voltage isvaried. Another aspect of the present invention provides that the flyingheight spacing of the slider body is detected based on detecting aminimum electrodynamic response of the slider to a first harmonic of theAC frequency of the AC component as the DC component is varied. Stillanother aspect of the present invention provides that the flying-heightof the slider body is detected based on a minimum interference betweenthe slider body and the hard disk at a frequency of the AC component asthe DC component is varied.

One embodiment of the present invention detects the flying-heightspacing of the slider body using a Laser Doppler Vibrometer. Anotherembodiment of the present invention detects the flying-height spacing ofthe slider body based on a magnitude of magnetic readback signal that issensed by a read element on the slider body at the frequency of the ACcomponent as a magnitude of the DC component is varied. For theseembodiments of the present invention, the AC component can be aswept-frequency AC signal or a single-frequency AC signal. Yet anotherembodiment of the present invention detects the flying-height of theslider body based on a minimum variation of current that flows on andoff the slider body as the DC component of the bias voltage is varied.Still another embodiment of the present invention detects theflying-height spacing of the slider body is based on a minimumresistance of a magnetoresistive element as the DC component is varied.A further embodiment of the present invention detects the flying-heightspacing of the slider body based on an output of a piezoelectric sensoror an acoustic emission sensor sensing contact between the slider bodyand the hard disk as the DC component is varied.

The magnitude of the predetermined voltage can be controlled based onthe detected flying-height spacing of the slider body, such as bydetermining a minimum slider-to-disk clearance change from the designflying height of the slider as the DC component is varied, or bydetermining a minimum interference between the slider body and the harddisk as the DC component is varied. Alternatively, the magnitude of thepredetermined voltage can be controlled so that an amplitude of avariation of the flying-height spacing times a sine of a phasedifference of the flying-height spacing at a frequency of the ACcomponent is a minimum.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and not bylimitation in the accompanying figures in which like reference numeralsindicate similar elements and in which:

FIG. 1 shows an exemplary hard disk drive;

FIG. 2 depicts an electrostatic field that can exist between a sliderbody and a hard disk of a hard disk drive;

FIG. 3 shows the flying height for two different air bearing designs asa function slider-disk potential difference;

FIG. 4 depicts the general concept for all embodiments of the presentinvention for determining the optimum DC bias voltage that should beapplied to a slider body for eliminating an electrical potentialdifference between a slider body and a hard disk;

FIG. 5 depicts a system using a Laser Doppler Vibrometer for determiningthe optimum bias voltage that should be applied between a slider bodyaccording to the present invention;

FIGS. 6A and 6B respectively shows the first and second harmonicresponse of an exemplary slider at the air bearing frequency range fordifferent DC bias voltages;

FIGS. 7A and 7B respectively show the first harmonic displacement andphase of an exemplary slider as a function of DC bias voltage at asingle AC frequency;

FIG. 8 depicts a first configuration of a third embodiment of thepresent invention for determining the optimum DC bias voltage thatshould be applied between a slider body and a hard disk;

FIG. 9 depicts an alternative configuration of the third embodiment ofthe present invention for determining the optimum DC bias voltage thatshould be applied between a slider body and a hard disk;

FIG. 10 shows a graph of the rms Voutput for the alternative currentsense circuit shown in FIG. 9 when R₁=8 MΩ and R₂=R₃=1 MΩ;

FIG. 11 shows a graph of resistance of an exemplary GMR sensor as afunction of applied DC bias voltage;

FIG. 12 depicts an exemplary piezoelectric pressure sensor that can beused for determining the optimum bias voltage that should be appliedbetween a slider body according to the present invention;

FIG. 13 shows the output of an AE sensor for an exemplary slider as afunction of an applied bias voltage having a DC component and aswept-frequency AC component; and

FIG. 14 depicts an exemplary control system for applying the optimum DCbias voltage between a slider body and a hard disk according to thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a technique for determining the optimumbias voltage that should be applied between a slider body and a disk foreliminating an electrical potential difference that exists between theslider body and the disk, such as a contact potential caused bytribocharging or by the material differences of slider and disk. Byeliminating the electric field between the slider body and the disk, theslider flying height is increased to the maximum design flying height ofthe slider. Removal and depletion of lubricant from the disk surfacecaused by electrostatic forces are also eliminated. Accordingly, becauselubricant is not removed from the disk, lubricant does not becomeconcentrated in areas on the slider body and lubricant droplets do notform. Consequently, read/write errors caused by lubricant droplets inthe head-disk interface are avoided. Further, vibrational couplingbetween the slider body and the disk is reduced because theelectrostatic force between the slider body and disk has beeneliminated, thereby eliminating slider-disk spacing modulations.

According to the present invention, a bias voltage having a DC componentplus a swept-frequency AC component is applied between a slider body anda disk, and the minimum electrodynamic response of the slider to thefirst harmonic of the swept-frequency AC component, which is preferablyis within the range of the air bearing (i.e., 50 kHz to 500 kHz), isdetected as the magnitude of the DC component is varied for determiningthe contact voltage between the slider and the disk. Alternatively, theAC component of the bias voltage has a fixed frequency, which ispreferably within the range of the air bearing (i.e., 50 kHz to 500kHz). The range that the magnitude of the DC component can be varied canbe about −5.0 V DC to about +5.0 V DC with a current limitation of about1 μA. The magnitude of the AC component should preferably not exceedabout 300 mV.

A first aspect of the present invention utilizes the slider body and ahard disk as a Kelvin probe to determine the optimum DC bias voltagethat should be applied between the slider body and the hard by detectingthe minimum electrodynamic response of the slider to the first harmonicof the AC frequency of the AC component of the bias voltage as the DCcomponent of the bias voltage is varied. The minimum electrodynamicresponse of the slider can be detected using an external Laser DopplerVibrometer (LDV) or laser interferometer and/or by monitoring themagnitude of a readback signal at the frequency of the AC component ofthe bias voltage. A second aspect of the invention provides that theoptimum DC bias voltage that should be applied between a slider body anda disk is determined by detecting the magnitude of the currentfluctuations that flow on and off of the slider as the DC bias voltageis varied. A third aspect of the present invention provides that theoptimum DC bias voltage that should be applied between a slider body anda disk is determined by detecting interference between the slider andthe disk. It should be understood that throughout the description of thepresent invention, phrases such as “maximum slider-disk spacing” or“maximum flying height of the slider” should be understood to mean thedesign flying height of the slider.

FIG. 4 depicts the general concept of all embodiments of the presentinvention for determining the optimum DC bias voltage that should beapplied to a slider body for eliminating an electrical potentialdifference between a slider body 401 and a hard disk 402. As disk 402rotates, disk 402 moves from right to left with respect to slider body401, as indicated by arrow 403. A suspension 404 supports slider body401. While disk 402 is indicated as being at ground potential, it shouldbe understood that disk 402 can have a potential difference relative tothe rest of the HDD or slider body, such as that caused by tribochargingof the spindle motor bearing. The embodiments would also work when theDC and AC voltages were applied to the disk rather than to the sliderbody. A DC bias voltage V_(DC) plus an AC bias voltage V₀ sin ωt areapplied to slider body 401 through suspension 404. At a given ACfrequency, the DC bias voltage is varied to determine the minimum amountof slider/disk interference.

The first two embodiments of the present invention utilize the sliderbody and the hard disk as a Kelvin probe to determine the optimum DCbias voltage that should be applied between the slider and the hard diskfor eliminating an electrical potential difference that exists betweenthe slider body and the disk, such as a contact potential caused bytribocharging. Generally, a Kelvin probe is a capacitor having one platevibrating at an AC frequency and the other plate fixed, similar to aslider body and a hard disk. The optimum DC bias voltage that should beapplied between the slider body and the hard disk is determined bydetecting the minimum electrodynamic response of the slider to the firstharmonic of the AC frequency of the AC component of the bias voltage asthe DC component of the bias voltage is varied.

To illustrate the Kelvin probe aspect of the present invention, considerthe electrostatic force F_(el) that acts between two parallel conductingplates formed from different materials, which is given by:$\begin{matrix}{{F_{el} = {\frac{ɛ_{0}ɛ_{r}A}{2d^{2}}V^{2}}},} & (1)\end{matrix}$in which ε₀ε_(r) is the permeability of the dielectric between theplates, A is the area of the plates, d is the distance of separation ofthe plates, and V is the potential difference between the plates.

When a slider-disk interface, such as shown in FIG. 4, is modeled by Eq.(1), the potential difference V between the plates, that is, the betweenslider and the disk, is, for the present invention, the sum of anexternally applied bias voltage and an internal contact potentialV_(contact). Contact potential V_(contact) is the potential differencebetween the two plates in the absence of an applied voltage andoriginates from the plates being made of different materials havingdifferent work functions. See, for example, N. W. Ashcroft et al.,Chapter 18, Solid State Physics, Holt, Rinehart and Winston, New York,1976. The work function of a material is the amount of energy that isneeded for releasing electrons from the surface of the material, and isrelated to the optical, electrical and mechanical properties of thematerial. When two materials having different work functions are broughttogether, electrons in the material having the higher work function flowto the material having the lower work function. When the materials aremade into a parallel plate capacitor, an equal and opposite surfacecharges form on each material. The voltage formed between the plates ofthe capacitor from the equal and opposite surface charges is called a“contact potential”. For slider-disk interfaces, the conducting part ofthe slider body is typically sintered Al₂O₃—TiC and the conducting partof the disk is typically a cobalt-based alloy magnetic layer. Therespective work functions for the materials of the slider body and theconducting part of the disk are further modified by overcoats andlubricants that are deposited for tribology protection.

When one plate (i.e., the conducting part of the disk) is grounded and abias voltage V_(bias)=V_(DC)+V_(AC) sin αx is applied to the other plate(i.e., the slider body), as shown in FIG. 4, the electrostatic force canbe written, after mathematical manipulation, as $\begin{matrix}{F_{el} = {\frac{ɛ_{0}ɛ_{r}A}{4d^{2}}\lbrack {{2( {V_{DC} + V_{contact}} )^{2}} + {4{V_{AC}( {V_{DC} + V_{contact}} )}\sin\quad\omega\quad t} - {V_{AC}^{2}\cos\quad 2\quad\omega\quad t} + V_{AC}^{2}} \rbrack}} & (2)\end{matrix}$

The first and last terms within the square brackets in Eq. (2) representthe DC response to the applied bias voltage V_(bias), the second termrepresents the first harmonic response to V_(bias), and the third termin square brackets represents the second harmonic response to V_(bias).The first harmonic term becomes zero when the DC component of theapplied bias voltage V_(bias) cancels the contact potential V_(contact),i.e., V_(DC)=−V_(contact). The second harmonic term, however, isindependent of the applied DC voltage. Thus, the optimum DC component ofthe applied bias voltage V_(bias) can be detected when the firstharmonic electrodynamic response of the slider to the AC component ofthe applied bias voltage V_(bias) equals zero (i.e., no slider vibrationat the first harmonic of the applied AC component).

The first embodiment of the present invention provides a technique thatis applicable when the disk drive is open. All of the other embodimentsof the present invention that will be described herein are applicablewhen the disk drive is sealed. According to the first embodiment of thepresent invention, a Laser Doppler Vibrometer (LDV) that is external tothe disk drive is used for determining the minimum slider-to-diskclearance change from the design flying height of the slider as theapplied bias voltages are varied. FIG. 5 depicts a system 500 formeasuring the flying height of a slider with respect to a disk using anLDV. In FIG. 5, a slider body 501 is suspended above a hard disk 502 bya suspension 504 as disk 502 moves from right to left with respect toslider body 501, as indicated by arrow 503. A bias voltage having a DCcomponent V_(DC) plus an AC component V₀ sin ωt is output from an AC/DCpower supply 706 and is applied between slider body 501 and disk 502.The bias voltage is applied to slider body 501 through suspension 504.The bias voltage is varied under the control of computer 507. An LDV 508measures a velocity of the trailing edge of slider body 501. A lockinamplifier 509 measures first and second harmonics of the LDV velocitysignal at the AC driving frequency, which is typically at an air bearingfrequency. The first harmonic of the AC driving frequency is minimizedwhen the DC bias voltage cancels the contact potential between sliderbody 501 and disk 502.

FIGS. 6A and 6B respectively shows the first and second harmonicresponse of an exemplary slider at the air bearing frequency range (50kHz to 500 kHz) for DC bias voltages of −0.5 V, 0 V and +0.5 V DC. Themagnitude of the AC component of the applied bias voltage for all DCbias voltages was 100 mV. Curve 601 represents the first harmonicresponse of the slider for an applied bias voltage having a DC componentof +0.5 V. Curve 602 represents the first harmonic response of theslider for an applied bias voltage having a DC component of 0 V DC.Curve 603 represents the first harmonic response of the slider for anapplied bias voltage having a DC component of −0.5 V DC. The firstharmonic response disappears when a DC bias of +0.5 V (curve 601) isapplied to the slider body, implying that V_(contact)=−0.5 V DC for theslider-disk interface. Moreover, curve 601 verifies that the slider-diskinterface behaves like a Kelvin probe. The curves representing therespective responses for the second harmonic of the applied bias voltageremain unchanged for the different applied bias voltages and areessentially indistinguishable from each other. Consequently, the secondharmonic responses are not separately indicated. Note that the secondpitch mode frequency at ˜320 kHz is strongly excited by the AC componentof the applied bias voltage, while the first pitch at ˜120 kHz is weaklyexcited.

Rather than sweeping through the all air bearing frequencies, thecontact potential could also be determined simply by monitoring thevalue of the DC component of the applied bias voltage and determiningwhen the electrodynamic response to the first harmonic of asingle-frequency AC component of the applied bias voltage is minimized.The frequency of the single-frequency AC component should be preferablyat a mechanical resonance, such as a suspension resonance, an airbearing resonance or a slider body resonance. For example, a highsignal-to-noise ratio is achieved when the frequency is near the pitch 2mode frequency of a slider air bearing. FIGS. 7A and 7B respectivelyshow the first harmonic displacement and phase of an exemplary sliderair bearing resonance as a function of DC bias voltage for a 290 kHz ACbias voltage having a magnitude of about 100 mV. FIG. 7A shows that theDC component of the applied bias voltage is about −0.4 V DC when theelectrodynamic response at the first harmonic of a single-frequency ACcomponent of the applied bias voltage is minimized.

According to the second embodiment of the present invention, the minimumslider-to-disk clearance change from the design flying height of theslider based on a relative magnitude of a magnetic readback signalsensed by the read element on slider body 401, shown in FIG. 4. That is,when slider body 401 is correctly positioned over a selected track, themaximum magnitude of the readback signal varies as the applied biasvoltages are varied. In particular, the maximum magnitude of thereadback signal varies inversely to the flying height of the slider.Thus, when the flying height of the slider is a maximum, that is, at thedesign flying height of the slider, the maximum magnitude of thereadback signal will be a relative minimum. When the flying height ofthe slider is less than the design flying height of the slider, themaximum magnitude of the readback signal will be greater than when theslider is at the design flying height. For high-pitch sliders, however,the flying height will be changed from the design flying height by about0.1 nm or less for a contact potential of between 0 V DC and 0.5 V DC.Consequently, a change in the magnitude of the readback signal is likelynot detectable. To obtain an improved Signal-to-Noise (SNR) for thereadback signal the amplitude of the readback signal must be filtered tobe exactly the frequency of the AC component. When a swept-frequency ACcomponent is used, then a frequency selective device, such as a lockinamplifier, can be used. When a single frequency AC component is used, anotch-pass filter at the frequency of the AC component can be used. TheDC component is then varied while the filtered readback signal ismonitored. When the amplitude of the filtered readback signal is aminimum, the DC component of the applied bias voltage equals the contactpotential. According to an alternative configuration of the firstembodiment of the present invention, the maximum slider-disk spacing canbe identified based on the Position Error Signal (PES) that is generatedin a well-known manner. For the third embodiment of the presentinvention, it is not necessary to use an AC component of the appliedbias voltage to determine the optimum DC bias voltage, although nullingthe AC component of the change in the flying height is easier to detectusing an AC component.

A third embodiment of the present invention provides that the optimum DCbias voltage that should be applied to the slider body is detected byidentifying the minimum current variation that flows on and off theslider as the DC bias voltage is varied. FIG. 8 depicts a firstconfiguration for the third embodiment of the present invention fordetermining the optimum DC bias voltage that should be applied to aslider body for eliminating an electrical potential difference between aslider body 801 and a hard disk 802. As disk 802 rotates, disk 802 movesfrom right to left with respect to slider body 801, as indicated byarrow 803. A suspension 804 supports slider body 801. While disk 802 isindicated as being at ground potential, it should be understood thatdisk 802 can include an electrical potential relative to the HDD.According to the third embodiment of the present invention foridentifying the optimum DC bias voltage, a DC bias voltage V_(DC) isapplied to slider body 801 through suspension 804 (AC voltage V₀=0) andthen is varied to identify the minimum variation in current that flowson and off slider body 801. A current sense circuit 805 is coupled toslider body through suspension 804. Current sense circuit 805 includesan amplifier 806 that is configured in an inverting topology withfeedback resistor R. The bias voltage is applied to the non-invertinginput of amplifier 806.

Assuming that the tribocurrent is negligible and that the slider-diskresistance is infinite, the current i is given by the relationship:i=(dC/dt)(V_(C)+V_(DC))+CdV_(C)/dt, in which C is the slider-diskcapacitance and V_(C) is the contact potential. When the disk workfunction is uniform, dV_(C)/dt≈0, and the current i comes mainly fromthe variation of slider-disk capacitance, which varies due to changes inflying height and pitch. When slider body 801 flies without accessing,the average capacitance is constant and dC/dt varies about zero, so therms modulation of the current is minimum when V_(C)=−V_(DC).

Current sense circuit 805 in FIG. 8 measures both the DC and ACcomponents of current i. An alternative configuration for the thirdembodiment of the present invention is shown in FIG. 9, which depicts aslider body 901, a hard disk 902 and a current sense circuit 905 formeasuring only the AC component in the variation in current i. As disk902 rotates, disk 902 moves from right to left with respect to sliderbody 901, as indicated by arrow 903. A suspension 904 supports sliderbody 901. While disk 902 is indicated as being at ground potential, itshould be understood that disk 902 can include an electrical potentialrelative to the HDD. Current sense circuit 905 includes an amplifier 906that is configured in an inverting topology with resistors R₂ and R₃setting the gain. A capacitor C₁ DC-isolates amplifier 907 from sliderbody 901 and bias voltage source V_(DC). Resistor R₁ isolates biasvoltage source V_(DC) from capacitor C,

FIG. 10 shows a graph of the rms Voutput for alternative current sensecircuit 905, shown in FIG. 9, in which R₁=8 MΩ and R₂=R₃₌₁ MΩ. From FIG.9, it can be observed that the rms current variation goes through aminimum when the DC component V_(DC) of the applied bias voltage is −0.6V_(DC).

The fourth and fifth embodiments of the present invention related todetermining the optimum DC bias voltage that should be applied between aslider body and a hard disk by detecting the least amount ofinterference between the slider body and the hard disk. For the forthand fifth embodiments, the AC component of the applied bias voltage canbe used to create a slider vibration that is sufficiently large toinduce contact between the slider body and the hard disk. When theslider is flying sufficiently close to the disk, for example, 1 to 2 nmabove the disk, the AC component of the applied bias voltage would notbe necessary to induce contact between the slider body and the harddisk.

A fifth embodiment of the present invention provides that the optimum DCbias voltage that should be applied between a slider body and a disk isdetected by identifying the maximum slider-disk spacing by using apiezoelectric (PZT) sensor. FIG. 12 shows a side view of an exemplaryarrangement of a slider body 1201, a suspension 1202 and a flexure 1203having an exemplary piezoelectric pressure sensor 1204 for sensingpressure that flexure 1203 exerts against suspension 1202 at a dimple1205 as slider body 1201 contacts a disk (not shown) at 1206.Piezoelectric pressure sensor 1204 is fabricated as an integral part offlexure 1203, such as disclosed by co-pending and commonly assignedpatent application Ser. No. ______ (Attorney Docket No. HSJ920030073US1)and patent application Ser. No. ______ (Attorney Docket No.HSJ920030230US1), both of which are incorporated by reference herein.Piezoelectric pressure sensor 1204 detects head-disk interference (HDI)when slider body 1201 contacts a disk (not shown in FIG. 12). When theflying height of slider body 1201 is maximum, that is, at the designflying height of the slider, the output of piezoelectric pressure sensor1204 is a minimum. According to an alternative configuration of thefourth embodiment of the present invention, an acoustic emission (AE)sensor can be used instead of a piezoelectric pressure sensor, in whichcase detection of the maximum flying height of the slider would occurwhen the output of the acoustic emission sensor is a minimum. FIG. 13shows the output of an AE sensor for an exemplary slider as a functionof an applied bias voltage having a DC component. Curve 1301 representsthe output of the AE sensor after 20 minutes. Curve 1302 represents theoutput of the AE sensor after 17 hours.

Note that the point of minimum resistance for the GMR sensor shown inFIG. 11 and the point of minimum output of an AE sensor are not asdistinctly defined as the detected point of minimum displacement fromthe design flying height that is shown in FIG. 7. Thus, the Kelvinprobe-based embodiments of the present invention provide a superiortechnique for determining the optimum DC bias voltage that should beapplied between a slider body and a hard disk than the embodiments ofthe present invention that are based on detecting a minimum interferencebetween a slider body and a hard disk.

The bias voltage applied between a slider body and a disk can becontrolled by a feedback loop that adjusts the DC component V_(DC) ofthe applied bias voltage to keep A sin θ near zero, in which A is theamplitude of the slider-disk spacing variation at ω and θ is the phasedifference between the slider-disk spacing variation at ω and the ACcomponent V₀ sin ωt of the applied bias voltage.

Further, a control loop that determines the optimum V_(DC) bias voltagethat should be applied between a slider body and a disk can be based onany of the five alterative embodiments of the present invention fordetermining the optimum DC bias voltage that should be applied to aslider body for eliminating an electrical potential difference between aslider body and a hard disk. FIG. 14 depicts an exemplary control system1400 for applying the optimum DC bias voltage between a slider body 1401and a hard disk 1402 according to the present invention. Control system1400 includes a sensor system 1405 that senses the electrodynamicresponse of slider body 1401 to an applied AC bias voltage and/or alevel of interference between slider body 1401 and hard disk 1402 usingany of the techniques described in connection with the presentinvention. Sensor system 1405 outputs a control signal that correspondsto the sensed electrodynamic response of slider body 1401 and/or thelevel of interference between slider body 1401 and hard disk 1402. Abias voltage source 1406 outputs a bias voltage in response to thecontrol signal that is applied to slider body 1404.

It should be understood that while the present invention has beendescribed in terms of a single slider body and a single disk, thetechniques of the present invention can be applied to each respectiveslider-disk interface of an HDD individually or collectively. That is,the optimum bias voltage for each respective slider-disk interface canbe detected separately. Alternatively, the optimum bias voltage for asingle slider-disk interface can be detected and used for generating abias voltage that is applied to each respective slider-disk interface.Accordingly, any of the techniques of the present invention fordetermining the optimum DC bias voltage that should be applied between aslider body and a hard disk described herein can be used in a controlloop to control the applied DC bias voltage. Further, the techniques ofthe present invention can also be applied to magneto-optical drives oroptical drives that contain a slider having an aperture/lens and arotating disk.

While the present invention has been described in terms of applying anAC/DC bias voltage to a slider body, it should be understood that thepresent invention works equally well when an AC and/or DC bias voltageare applied to the disk instead of the slider body. Further, while thepresent invention has been described in terms of varying the DC biasvoltage from a negative value to a positive value, the range over whichthe DC bias voltage could be varied could be unipolar, in which case theoptimum DC bias voltage would be determined by linearly extrapolatingthe detected deviation from the design flying height of the slider tofind the intercept with the x-axis.

Although the present invention has been described in terms ofdetermining an optimum bias voltage that should be applied between aslider body and a disk for eliminating an electrical potentialdifference that exists between the slider body and the disk, it shouldbe understood that the Kelvin probe aspects of the present invention canbe used for detecting the condition or state of a slider-disk interfaceas it varies over time. For example, the optimum bias voltage can bedetermined using any of the embodiments of the present invention andused as a baseline against which subsequent detected changes in theoptimum bias voltage are used for determining, for example, the amountof lubricant that is present at different areas of the disk.Furthermore, because the contact potential is sensitive to fermi-levelchanges and to changes of the surface potential, the present inventioncan be used to monitor lubricant degradation or drive contamination.

Although the foregoing invention has been described in some detail forpurposes of clarity of understanding, it will be apparent that certainchanges and modifications may be practiced that are within the scope ofthe appended claims. Accordingly, the present embodiments are to beconsidered as illustrative and not restrictive, and the invention is notto be limited to the details given herein, but may be modified withinthe scope and equivalents of the appended claims.

1. A method for eliminating an electrical potential difference between aslider body and a hard disk of a hard disk drive, the method comprisingsteps of: detecting a flying-height spacing of the slider body betweenthe slider body and the hard disk; and applying a predetermined biasvoltage between the slider body and the hard disk, the predeterminedbias voltage including a DC component and an AC component and beingbased on the detected flying-height spacing of the slider body.
 2. Themethod according to claim 1, wherein the step of detecting theflying-height spacing of the slider body includes a step of determininga minimum slider-to-disk clearance change from a design flying height ofthe slider at a frequency of the AC component as the DC component of thepredetermined bias voltage is varied.
 3. The method according to claim1, wherein the step of detecting the flying-height spacing of the sliderbody includes a step of detecting a minimum electrodynamic response ofthe slider to a first harmonic of the AC frequency of the AC componentas the DC component is varied.
 4. The method according to claim 1,wherein the step of detecting the flying-height spacing of the sliderbody includes a step of detecting the flying-height spacing of theslider body using a Laser Doppler Vibrometer.
 5. The method according toclaim 1, wherein the step of applying a predetermined bias voltagebetween the slider body and the hard disk includes steps of: sensingusing a read element on the slider body a magnetic readback signal at afrequency of the AC component as a magnitude of the DC component isvaried; and detecting the flying-height spacing of the slider body basedon a magnitude of the detected magnetic readback signal.
 6. The methodaccording to claim 5, wherein the AC component is a swept-frequency ACsignal.
 7. The method according to claim 5, wherein the AC component isa single-frequency AC signal.
 8. The method according to claim 5,wherein the detected magnetic readback signal is a Position Error Signalrelating to a position of the slider body with respect to the hard disk.9. The method according to claim 1, wherein the step of detecting theflying height spacing of the slider body includes a step of determininga minimum interference between the slider body and the hard disk as theDC component is varied.
 10. The method according to claim 1, wherein thepredetermined bias voltage is applied to the slider body with respect tothe hard disk.
 11. The method according to claim 1, wherein thepredetermined bias voltage is applied to the hard disk with respect tothe slider body.
 12. The method according to claim 1, wherein the stepof applying the predetermined bias voltage includes a step ofcontrolling a magnitude of the predetermined voltage based on thedetected flying-height spacing of the slider body.
 13. The methodaccording to claim 12, wherein the step of detecting the flying heightspacing of the slider body includes a step of determining a minimumslider-to-disk clearance change from a design flying height of theslider at a frequency of the AC component as the DC component is varied.14. The method according to claim 12, wherein the step of detecting theflying height spacing of the slider body includes a step of determininga minimum interference between the slider body and the hard disk at afrequency of the AC component as the DC component is varied.
 15. Amethod for eliminating an electrical potential difference between aslider body and a hard disk of a hard disk drive, the method comprisingsteps of: applying a variable DC bias voltage between the slider bodyand the hard disk; detecting a minimum variation of current that flowson and off the slider body as the DC bias voltage is varied; andapplying a DC bias voltage corresponding to the variable DC bias voltageapplied between the slider body and the hard disk when the minimumvariation of current that flows on and off the slider body is detected.16. A method for eliminating an electrical potential difference betweena slider body and a hard disk of a hard disk drive, the methodcomprising steps of: detecting a level of interference between theslider body and the hard disk; and applying a predetermined bias voltagebetween the slider body and the hard disk, the predetermined biasvoltage including a DC component and being based on the detected levelof interference between the slider body and the hard disk.
 17. Themethod according to claim 16, wherein the slider includes amagnetoresistive element, and wherein the step of detecting the level ofinterference between the slider body and the hard disk is based on astep of detecting a minimum resistance of the magnetoresistive elementas the DC component is varied.
 18. The method according to claim 16,wherein the step of detecting the level of interference between theslider body and the hard disk is based on a step of detecting an outputof a piezoelectric sensor sensing contact between the slider body andthe hard disk as the DC component is varied.
 19. The method according toclaim 16, wherein the step of detecting the level of interferencebetween the slider body and the hard disk is based on a step ofdetecting an output of an acoustic emission sensor sensing contactbetween the slider and the hard disk as the DC component is varied. 20.The method according to claim 16, wherein the predetermined bias voltageis applied to the slider body with respect to the hard disk.
 21. Themethod according to claim 16, wherein the predetermined bias voltage isapplied to the hard disk with respect to the slider body.
 22. The methodaccording to claim 16, wherein the step of applying the predeterminedbias voltage includes a step of controlling a magnitude of thepredetermined voltage based on the detected level of interferencebetween the slider body and the hard disk.